1. Field of the Invention
This invention relates to an ultrathin stainless steel foil suitable for etching, more particularly to a stainless steel foil that is not readily susceptible to the voids (tunnel-like erosions) that have occasionally occurred during etching together with cracking in the rolling direction from the edges. Fields in which the stainless steel foil of the present invention are utilizable include machine components (precision microsprings, precision vibrator plates, printer types, cutting dies etc.) and electronic equipment components (hard disk suspensions, printed circuit boards, picture tube shadow masks, lead frames etc.)
2. Description of the Related Art
Etching is extensively applied industrially in the precision processing of metal materials for electronic equipment. It is an indispensable production technology particularly for hard disk suspensions, printed circuit boards, picture tube shadow masks, IC lead frames and the like. Moreover, the push toward miniaturization, lighter weight and longer service life in electronic equipment of recent years is expanding applications for stainless steel foils, especially ultrathin foils of steels excellent in corrosion resistance of thickness on the order of 20 .mu.m.
In current industrially utilized etching methods, the technique used to increase production efficiency is generally to conduct product shaping by dissolving the metal in an aqueous solution having highly corrosive ferric chloride (FeCl.sub.3) as main component. Specifically, the front and rear surfaces of the foil are formed with photoresist films only at the portions of the steel to be left undissolved and the unneeded portions are dissolved by spraying with the etchant.
From the technical viewpoint, the important point in this technology is to achieve uniform dissolution over the entire surface and to accurately dissolve the foil along the contour of the photoresist film. Particularly when making a very small component from ultrathin stainless steel foil, the occurrence of voids or cracks from the edges degrades the strength of fine portions to the point that they break during bending or other such working, or during use of the equipment in which they are used. As electronic equipment is required to have high reliability, fine voids and/or cracks are extremely harmful defects that degrade the service life and reliability of the equipment as a whole. Even a microscopic void or crack measuring only several tens of .mu.m in length can be a fatal defect in the case of an ultrathin foil of around 20 .mu.m thickness.
It has been pointed out that shaping imperfections occur during etching because of lack of metal structure uniformity and presence of component segregations and nonmetallic inclusions. In particular, it is known that nonmetallic inclusions become void starting points and that to minimize them is effective for improving product dimensional precision. Japanese Unexamined Patent Publication No. 60(1985)-92449 teaches control of the number of pits produced by etching to 30 pits/15 mm.sup.2 by using a martensitic stainless steel for etching that satisfies the relationship of (S ppm)&lt;42-0.094 (O ppm), where S and O contents (mass) are expressed in ppm units. This reflects the fact that to prevent dissolution of inclusion it is necessary to regulate the amount of sulfide inclusions in proportion to the amount of oxygen because soluble sulfides like MnS readily precipitate with oxide inclusions as nuclei.
This technique of avoiding shaping defects by reducing sulfide inclusions has also been applied to ordinary steels. Japanese Unexamined Patent Publication No. 60(1985)-103153 teaches a quenched and annealed carbon steel having an S content of 0.004 wt %, while Japanese Unexamined Patent Publication No. 7(1995)-268541 teaches an ordinary steel regulated to a content of S, an impurity, of 0.002 wt %.
In the case of recent ultrathin stainless steel foils of a thickness on the order of 20 .mu.m, however, restriction of S and/or O to the aforesaid ranges of the prior art does not enable avoidance of voids and/or cracks extending in the rolling direction from the edges formed by etching.
In the prior art examples, the local corrosion during etching produces etching pits, hemispherical recesses, craters and other such depressions that are smaller in depth than the opening width. In the case of ultrathin foil, however, the local corrosion produces tunnel-like voids that are 2-5 .mu.m in inside diameter and extend to a length of more than 10 .mu.m in the rolling direction. This is a different phenomenon for which no solution has been known.